Electrical wave filter



Nov. 12, 1940.- B. TRI-:VOR 2,220,922

ELETRICAL WAVE FILTER Filed March 17, 193s 5- shets-sheet 1 BERTRAM'TREVOR .gy 7 MIM/V ATTORNEY.

Nov. 12, 1940. B. TREVQR ELECTRICAL WAVE FILTER 5 SheeLs-Sheet `Filed March 1'7, 1958 /30 l 440 FREQUENCY/N MEAcYcLES INVENTOR. ERTRAM TRE VOR AT'IORNEY.4

Nov. 12, 1940; l EL TREvoR 2,226,922

ELECTRICAL wAvE FILTER Filed March 17, 1938 5 sheets-shag s F/LTER FILTER TERM/NMS TERMINA Ls L 20g zc, F/UER l r FILTER TERM/HALS L2 L C2 TERM/MmsV IENTOR. TRAM ms/012 ATTORNEY.

Nov 12, 1.940. E. TRI-:VOR

"ELECTRICAL WAVE FILTER y 5 Sheets-Sheet 4 Filed March 17, 1938 /l//l//l INVENTOR. BERT? TREVOR ATTORNEY. l

Nov. 129 l94.

B. TRI-:VOR

ELECTRICAL WAVE FILTER Filed March 17, 1938 5 Sheets-Sheet 5 /NPUT INPUT INPUT wiwi? I 'AND b'cnMB/Nfo I NV EN TOR. BERT/QAM me von ATTORNEY.

Patented Nov. 12, 1940 UNITED STATES ELECTRICAL WAVE FILTER Bertram Trevor, Riverhead,` N. Y., assignor `'to Radio Corporation of America, acorporaton of Delaware Application March 17,

18 Claims.

This invention relates to band-pass electrical wave filters employing concentric line resonators- "as tuned circuitsfand particularly to band-pass filters suitable for passing Wide band television signals at ultra high frequencies.`

In the radio eld itis often desirable to operate a transmitter and receiver in close proximity, and with as small a frequency separation as possible Without introducing transmitter interference in ,0 the receiving system. To do this requires excellent selectivity preceding the first receiving tube to prevent cross modulation products from appearing in the receiveroutput.` In the case of wide band systems at ultra high frequencies it is 15' diicult with conventional methods to obtain the required dat band-pass along with sharply rising i attenuation on each Sider of the band-pass. Y

The present invention provides a more effective way to accomplish this desired result'and has for one of its objectsv to provide a band-pass lter having uniformly low attenuation over a desiredV band of frequencies, sharply rising attenuation at the edges of the passeband, and high attenuation above and below the pass-band. Another 25 object is to, provide a veryeflective shielded lter for use at ultra high frequencies, the filter to 'be such that trial and error are essentially eliminated in the iinal adjustments. y

The design of band-pass filters employing 30, coiled inductance is well knownk to the art and it is also knowny that the losses in the filter elements impose restrictions upon the attenuation 'versus frequency characteristic that may be obtained with any given network. For this reason, a dis- 35 tinct advantage is obtained by using, in accordance with the invention, tuned circuits in the form of concentric line resonators asy lter elements having very low loss,l along with capacitances having comparablelosses. In the frequency 40 range from 100/ megacycles to 200 ,megacyclea concentric line resonators can easily be built to have a Q (ratio'of reactance to resistance) of 1000 to 10,000, depending upon the diameters of conductors used. Furthermore, concentric line 45 resonators maybe totally enclosed, giving essentially perfectA shielding from outside fields, which is ,a necessary requirement wheny the filter is located near a transmitter.

As is known, a concentric line resonator com- 50 prises an outer conductor and a coaxial inner conductor, both conductors7 being coupled in such fashion that the inductance of the conductors together with the capacitance between the` same combine to form a resonant circuit whose res.- 55, onant frequency is determined to a large extent 193s, serial No. 196,362

(ci. 17e-44) by the dimensions of the conductors. One such concentric resonant line` consists of two concentric conductors conductively coupled together at one of vtheir adjacent ends and capacitively coupled together at their other ends. Examples of such concentric line resonators are described in British Patent No. 460,118, dated January 2l, 1937 United `States Patent No. 2,l02,805,`granted December 2-1, 1937; United States Reissue Patent No. 20,139, granted December 1, 1936; and in the article by Clarence W,v Hansell entitled Resonant lines for frequency controlj" published in Elec trical Engineering, August, 1935, pages 852-857, to which referencev is herein made` for a more completedescription of the general operation of concentric line` resonators.

In brief, the present invention relates to the class `of rlters generally described in my cepending application Serial No. 191,424, filed' February i9, 1938, and concerns improvements thereover.

Other objects, features andl their advantages will appear from a reading of the following detailed description of the invention, which is accompanied by drawings wherein: 1 Fig. lillustrates,` by vWay of example1 only, a band-pass lter constructed in accordance with the principles5 of the present invention;

Fig. la `shows the circuitwhich is electrically equivalentl to the filter of Fig. 1;

Fig. 2 graphically illustrates the `attenuation versusfrequency characterstic of the filter of Figa-1; i f

Fig. 3 shows a filter section, constructed of concentric resonators, having unequal terminal impedances, one terminal ofvvhichcan be made to have the same characteristic `impedance as the iilterof Fig. 1; i

Fig. 4 shows the circuitwhich is electrically equivalent. to the lter of Fig. 3;

5 shows aiilter arrangement in accordance with `the invention, of the mid-series, mi-derived type; i l

Fig'. G'shows the circuit which is electrically equivalent to .the filter of Fig. 5;

Fig. 7 illustrates another form of lter of the invention, which combines some" of the features ofFigs. 1 and 3;

Figs. 8 andf9 illustrate' lter circuits in accordance with `the invention, for obtaining a desired selectivity` between` radio frequency vacuum tube stages;- f Fig... 10: shows aL iilter circuit` in accordance with the rinvention which is equivalent to that of Fig'. 3; y

Fig 11 shows a cascade filter arrangement comprising the combination of Figs. 1 and 10;

Fig. 12 shows the circuit which is electrically equivalent of the filter arrangement of Fig. 1l; and

Fig. 13 is a circuit similar to that of Fig. 12 wherein the two inductances a' and b are combined to form a single inductance.

Referring to Fig. 1, there is shown a band-pass iilter composed of two similar sections or halves which are electrically coupled together, the cor-r responding elements of which have the same reference numerals, except that the referencenumerals of one section vor half have a prime designation. Each section or half is composed of two concentric line resonators, one contained within the other. Because the two halves of the filter of Fig. 1 are similar in their mechanical construction, it is deemed necessary to describe only one of these, let us say the lower half of the filter as it s shown in the drawings. l

The outer concentric line resonator of the lower half of the iilter of Fig. 1 comprises an outer metallic cylindrical conductor I and an inner conductor consisting of two metallic cylindrical, coaxial, conductors 3, 2 of different diameters. Conductors 2 and 3 are hollow and are connected together at their adjacent ends by metallic end plate 9. The other end of conductor 2 is closed by metallic end plate 5, while the other end of conductor 3 is directly connected to one end of cylinder I by means of metallic end plate I 0. The other end of cylinder I is closed by an end plate II, in turn connected to and supporting, by means of bolts 24 and nuts I2, a circular plate 4, the latter forming a capacitance with plate 5 from which it is spaced. Plate 4 is iitted around its periphery with spring contacts I4 to make a positive low resistance connection to conductor I. Movement of plate 4 with a consequent adjustment of the capacity between plates 4 and 5 and variation of the resonant frequency of the resonator, is achieved by turning the adjusting nuts I2. Springs I3 serve to force plate 4 toward plate 5 upon unscrewing nuts I 2. Cylinders I, 2, 3 and end plates 4, 5, 9 and I0 are made of a highly electrically conductive material, such as copper, or have their surfaces coated with such a material.

In effect, the outer line resonator I, III, 3, 9,

2, 5, 4 is a tuned circuit; the small diameter section 3 may be considered to form with the outer conductor I an elective inductance, and the larger diameter section 2 may be considered to form with the outer conductor I an effective capacitance. The entire effective capacitance of the outer concentric resonator is formed by the capacitance between the cylindrical conductors I and 2, and by the capacitance between end plates 4 and 5. Such an arrangement is generally described in' United States Patent No. 2,124,029, granted to James W. Conklin et al. July 19, 1938, to which reference is made for a complete technical'description of this particular type of two-diameter inner conductor concentric line resonator.

The inner concentric line resonator of the lower half of the iilter of Fig. 1 is contained withincylinder 2 and consists of an inner cylindrical coaxial rod 8 which is directlyconnected at 4one end to the inner-surface ofk cylinder 2 by end plate 9, and capacitively connected at its other end tothe cylinder 2 by means of circular plates I and 6. Plate ,'I yis directly connected to rod' 8, while plate 6 is supported on plate5 and suitably spaced away from plate 1. Plate 6 is provided with a spring contact I1 at its periphery to make a low resistance connection with cylinder 2, and is adjustable in position relative to plate 1 by means of nuts I5 and bolts 25. Springs I6 serve thefsame purpose for the inner resonator as do springs I3 for the outer resonator. All plates and cylinders are made of material having high electrical conductivity.

In effect, the inner line resonator 2, 9, 8, l, 6 is a tuned circuit. The conductor 8 forms with the cylinder 2 an eiective inductance, while the spacing between plates 6 and I forms an eifective capacitance. There is also a small capacitance between the edge of plate 1 and cylinder 2.

The two halves of the lter of Fig. 1 are connected together by means of a connector I9 supported at each end by pin jacks I8 and I9. At this location, a small vamount of stray unwanted coupling appears between the two halves of the iilter, and for this rea-son there are provided the metallic sleeves 20 and 20 to prevent connector I9 from being capacitively coupled to ground. Such a capacitive coupling to ground would be somewhat detrimental to the normal operation of the iilter. The addition of the sleeves 20 and 2U materially reduces the capacitance between conductor I9 and ground and adds a small capacitive coupling between conductors 2 and 2. In most cases, this small coupling does not materially affect the proper operation of the lter. The term ground as herein employed refers to any of the external conductors I, I0, II, 4, I', I0', II', 4 which may be at zero or a relatively fixed alternating current potential.

The input terminals to the iilter of Fig. 1 are 2I and 22, while the output terminals are 2| and 22. Apertures are provided in the outer cylinders I and I near terminals 2I and 2| to enable connection from external circuits to conductors 2 and 2, respectively, at terminal points 22 and 22'. It should be understood, of course, that the input and output terminals may be used interchangeably, and terminals 22 and 22' maybe located at any point along the length of cylinder 2 and 2 respectively. v

A better understanding of the filter of Fig. 1 may be had by referring to the electrically equivalent circuit shown in Fig. 1a, which, it Will be evident, is the familiar mid-shunt section of an '1n-derived type of band-pass iilter. The theory underlying this type of lter section is well known and is described by Shea in his book entitled Transmission Networks and Wave Filters, published in 1929 by D. Van Nostrand Company, Inc., New York. The constants of the lter of the invention are designed to conform with well known practices, as set forth for example by Shea on page 315 of the above mentioned book.

In Fig. la, the elements shown to the left of the vertical dot and dash line are contained within the cylindrical conductor I, of Fig. 1, and the elementsto the right of the dot and dash line are contained within the cylindrical conductor I of Fig. 1. The shunt input inductance 2L2 is formed by the cylindrical conductor 3 located within and coaxial with respect to conductor I. The shunt input capacitance Cz/z is formed by the capacitance between the cylindrical conductors I and 2, and the circular end plates 4 and 5. The series elements L1 and C1 are contained entirely within conductor 2. The inductance L1 is formed by the cylindrical conductor 8 located within and coaxial with respectl lteri'ofthe present invention to-give any desired` to' conductor 2.

Cs of Fig. 1a differ from the values of L1. and C1.

respectively; as required by a particular design of the filter. It should be noted in this connectionv that elements 8' and `2 of Fig. 1 are slightly longer than the' corresponding elements 8` and 2.

It should be observed that the construction of y the lter of` the invention is such that theY elements L1, C1 and lo, C3 are completely enclosed within their respective conductors 2` and 2", and

consequently are completelyisolated, `both ca-` pacitively and inductively, from any otherelement `in the filter. or from ground.` In similar manner, veach element of the filter has nof undesired coupling to any other element of the frite except as mentioned above. l l

Fig. 2 graphically illustrates the" measuredat- 1 tenuation Versus frequency curve ot theA iter of Fig. 1. vIn Fig. 2, the ordinaterepresents attenuation in decibels, while the abscissa represents frequency in megacycles. Aninspection of this curve will show that the band-pass lter of Fig. 1 has substantially zero attenuation over the band of frequencies from about 127 megacycles to -133 megacycles; sharply rising attenuation at the edges of the pass band, and high attenuation above and below'the pass band. Inthe filter of Fig. i, for which Fig. 2 is the curve, fm is 139' mc.; f1 is 126.0 mc.; f2 is 134.13 mc.; 1` is 121.1 mc.;

fam is 139.4 mc.; R is 100 ohms, and 1r1.`is10.`895,`

`where f1 and f2 are thelower and upper nominal out off frequencies, fi and fz `are the lower and upper frequencies at which `maximumat tenuation occurs, fm is equal to l and '\/f1DOf200s R is the characteristic impedance atlmid-band, andmisequalto 'l ,y

matched. In such a case the outer conductor of theinput transmission line would.` connect to con-- ductor l at point 2l and the inner vconductor of the transmission line would connect `to conductor `2 at point 22. Similarly, the output transmission line would connect to points 2l and 22'. Of

jcourse, the filter can be used in lother arrangements, for example, between rany two resistive impedances, of any sort whatever, having the'correct value.

One or more frlter sections of the present invention, may be cascaded togive increased attenu-` ation outside of the pass-'band and a sharper rise of attenuation at` the edges of 4the pass-band. Also, any other type of filter section having the same image impedance may be cascaded with the The capacitance C1 is formed. `bythe capacitance between circular platesv B'Iand characteristic t'hatiti's possible to obtain. For

example,.the. filter section shown in Fig. 3, con` structed. of. concentric line resonators 2$ and21,`

may be builtlto have unequalterminalimpedances one of which, let us say 26, will be similar to and willmatch thati of lthe filter shown.` in Fig.` 1. The

othenfor outputterminal impedance of` the; secf` tion of..Fig'."3-, let us say 21, may be made higher or lowerthan its input terminal impedance.. In

- thisway; the filter sections of Figs. Land-3 maybe means of acondenser 2st located, at leastpartly, within` the outer conductor of resonator 21. They f outer conductors of both line resonators are maintained atlzero` orv a iixed alternating current potenti-al by fmeans of ground connections 29`and` 30.A V,Input terminals connect, by way of example only,V to theouter conductor of resonator 26 and thehigh potential end of the inner conductor ofv this same resonator. Output terminals are shown connected tothe outer conductor of resonator 21 andto the high potential end of the inner con-` ductor of the same resonator. The section shown (Fig. 3) ,provides a step-up of impedance which might be used in` conjunction with the` lterof.

Fig. 1 to provide a network to give a desireddef` gree `of selectivity between` a coaxial transmission l line and the gridelectrode of a therrnionic amplier or detector vacuum tube. i l

Fig. 4 shows the circuit which isr electrically equivalent totheilter oi Fig. 3. The elements of Fig. Alltxshown to" the left of the verticaldash linecomprise the concentric line resor'iatorl 26, while thehelements to the `right of the vertical dash line, exceptI for the coupling capacitor 28, com-` prise the concentric line resonator 21. The theory underlying the type of lter shown in Fig.` e is well known. and" is described in the book by Shea.,

Supra l l, Fig. 5 illustrates, schematically, a nner embodi-l ment of theinvention, making use of the midy serieswsection of the m-derived lter. 4.The circuit which is electrically equivalent to the filter of Fig. 5` is shownin Fig. 6. A better understanding of `Fig.5 may be had by referring Yto `theconventional electrical equivalent circuit of Fig. 6.. Theisame symbolshave been used in, bothfof. these figures to designate 4equivalent. element'sf In effect, Fig. 5` consists` of five tuned circuits in the `form of ve concentric line resonators, all contained, within` the hollow metallic container 3l., YThe shunt line resonator (corresponding to.

L2, C2` of Fig. 6) comprises thetwo-diameter in` ner conductor 32, 33 andthe outer conductor 31. close spacing to conductor 3`I forms an eiective capacitanceA therewith.` 'Ihe interior ,of hollow conductor 33 is separated into two chambers A and B` by` means of a metallic partitionwall 34. In each of these two chambers` are two other concentric line resonators, one resonator of` which comprises the inner conductor 35, 36 of two diameters; and an `outer conductor consisting of Conductor 33 is hollow. and by virtue of, its` in Fig. 1. The adjustments of .the .resonant frequencies of the individual line resonators'may in Fig. 1, or ini l vbe made -as shown and described any other suitable fashion.

An inspection of the einer ofrig. 5 lWin Asnowr Fig. 7 shows a band-pass lter in accordance with the invention, wherein'certain features of the filters of Figs.l 1 and 3 are combined.

If it is desired to supply selectivity between two radio frequency vacuum tubes,VVV use may bemade of two sections similar Vto that of Fig. 3, one preceding and one following the lter of Fig. 1 to allow a step-downV from the 'plate circuit ing grid circuit of the other' vacuumy tube, as shown in Fig. 8. The circuit of Fig. 8'can be simplified to that shown in Fig. 9 bycombining' adjacent parallel resonators into a single reso nator having proper constants methods well known to the'art.

In Fig. 10 is shown a filter section equivalent 30. ldetermined by to that of Fig. 3. This equivalence is described.

in my copending application, Serial No. 191,424, supra. Fig. 1l showsthe lter of Fig. 1 cascaded with that of Fig. 10. Fig. 12 shows the equivalent electrical circuit of Fig. 11, and Fig. 13 shows vthe equivalent electrical circuit of Fig. 12, wherein the two inductances a', b' are combined to forma single inductance as shown. The cirf 4o cuit (Fig. 13) is also the electrical equivalent rof the arrangement of Fig. l'7 in which the outputV shunt elements of the m-derived section are,

combined with the input shunt elements of the lter vof Fig. 10 to form a single LC circuit. AA

combination of the sort describedA above gives,

in effect, a lter having 'certaindesired selectivity characteristics as well as providing a means of transforming from one impedance value to another. Such impedance transfers are desirable to couple a low impedance transmission line to the grid of a tube while at the same time giving the selectivity provided by the particular filter combination used. What is claimed is: j j o l. A'band-pass electrical wave filter comprising a rst resonant circuit composed of asectionof coaxial transmission line having a hollow metallic enlargement' at one end of the inner` conductor to form a Vlumped tuning capacitance, a second resonant circuit located within said metallic enlargement and coupled thereto; 'a

similar arrangement of third and fourth reso-y nant circuits,` and means coupling together -said two arrangements.

2. A band-pass electrical wave filter compris- 4ing aresonant `circuit composed of a section .of

coaxial li,'ransrnis sim line employing a metallic enlargement atone end of the inner conductor to form a lumped tuning capacitance, anoth'erfresvo-A nant circuitfhcomposedf of a section of coaxial transmission line of *different dimensions-than the section of lineof said rst resonantA circuit,v

and means comprising still a third resonant circ uit composedfof awsection lof coaxial transmis-f of one vacuum tube and a step-up to the follow-l each of chambers A and B is substantially'siini` lar in construction to one-half of thelter shown sion line .located within Nsaid metallic enlarge# mentland coupling said rst and second resonant circuits together.

3. A band-pass electrical wave lter comprising a'rst resonant circuit composed of a section offcoaxial transmission line having a hollow metallic enlargement Vat one end of the inner conductor toform a lumped tuning capacitance, a second resonant circuit located within said metallic venlargement and coupled thereto; a similar arrangement of third and fourth resonant circuits, and means coupling together said two arrangements, 'said resonant circuits being so constructed land coupled together as to provide a filter having uniformly low attenuation overa desiredband of frequencies, sharply rising attenuation at'the, edges of the pass-band, and high attenuation above and below the pass-band. 4. A band-pass electrical wave filter comprising a first resonant circuit composed cf a section of coaxial transmission line having a hollow metallic enlargement at one end of the inner conductor to form a lumped tuning capacitance, a second resonant circuit located within said metallic enlargement and comprising an inner conductor and an outer conductor, the latter of which is the inner surface of said enlargement, a similar arrangement of third and fourth resonant circuits, and a `conductive connection between the inner conductors of said second and fourth resonant circuits.

5. A band-pass electrical wave filter of the 'rn-derived type comprising shunt and series resonant circuits, said shunt resonant circuit being composed of a section of coaxial transmission line having a hollow metallic enlargement at one end of theinner conductor to form a lumped tuning capacitance, said series resonant circuit being located Within said metallic enlargement and comprising an inner conductor which cooperates with the inner surface of said enlargement to form a tuned circuit in the form of a section of coaxial transmission line.v I 6. A band-pass electrical wave lilter comprislng' a shunt and a plurality of series resonant circuits, said shunt resonant circuit being composed of a section of coaxial transmission line having a metallic enlargement at one end of the inner conductor to form a lumped tuning capacitance, a partition dividing said metallic enlargement into two chambers, and a series resonant circuit located in each chamber, each of said series resonant circuits comprising an inner conductor which cooperates with the inner surface of its associated chamber to form a tuned circuit 1in the forni of a section of coaxial transmission 7. Aband-pass electrical wave filter comprising a shunt and a plurality of yseries resonant circuits, said shunt resonant circuit being composed of a section of coaxial transmission line having a metallic enlargement" at one end of the inner conductor to forma lumped tuning capacitance, a partition dividing said metallic enlargement into two chambers, and two series resonant circuits located'in ea'ch chamber, one of said series l tuned circuits in each chamber comprising an inner conductorwhich cooperates with the inner surface of its associated chamber to form al tuned circuit in the form of a section of coaxial transmission line, said last inner conductor also having a hollow metallic enlargement at one end to form a lumped 'tuning capacitance, the other series resonant circuit in each chamber being located' within the enlargement of the inner conductor of the first series resonant circuit in said chamber.

8. The combination with an input circuitk and an output circuit, of a band-pass lter connected between said output and input circuits, said lter providing a step-down in impedance from said output circuit and a step-up in impedance to said input circuit, said iilter including at least two cascaded lter sections each of which is composed of a resonant circuit comprising a portion of a coaxial transmission line having a metallic enlargement at one end of the inner conductor to form a lumped tuning capacitance, and another resonant circuit also comprising a portion of a coaxial transmission line, and means for capacitively coupling the `inner!` conductors of both resonant circuits of each iilter section together,`

there being` a connection provided for coupling together said metallic enlargements of both filter sections.

9. The combination with an input circuit and an output circuit, of a band-pass lter connected between said output and input circuits, said iil'ter` providing a step-down in impedance from said output circuit and a step-up in impedance to said input circuit, said lter including at least two cascaded filter sections each of which is composed of a resonant circuit comprising a portion of a coaxial transmission line having a metallic enlargement at one end oi the inner conductor to form a lumped tuning capacitance, and another resonant circuit also comprising a portion oi a coaxial transmission line, and means for coupling the inner conductors of `both resonant circuits of each filter section together, there being a connection provided for coupling together said metallic enlargements of both iilter sections.

10. Aband-pass lter in accordance with claim 1, characterized in this that the inner resonantcircuit of either section of transmission line is a coiled inductance having suitable tuning capaci-` having a hollow metallic enlargement at one end thereof to form a lumped capacitance, said sec- ,t ond tuned circuit being located completely within said metallic enlargement, wherebcr the inner surface of said metallic enlargement forms the` outer conductor of said second tuned circuit.

13. An electrical wave filter comprising a rst resonant circuit composed of a section of coaxial transmission line having a hollow metallic enlargement at one end of the inner conductor to form a lumped capacitance, and a second resonant circuit located completely :within said metallic enlargement and composed of a section of coaxial transmission line having an inner concircuit.

ductor of uniform longitudinal dimensions, a capacitive connection between said hollow metallic enlargement of said first resonant circuit and said inner conductor of uniform longitudinal dimensions of said second resonant circuit, said hollow enlargement and its associated outer conductor comprising one set of terminalsfor said filter, and said inner conductor of uniform dimensions and its associated outer conductor comprlsing another set of terminals for-said filter.

`14. An electrical wave lter comprising a iirst resonant circuit composed of a section of coaxial transmission line having a hollow metallic enlargement-at one end of the inner conductor to form a lumped capacitance,-and a second `resonant circuit composed of -a section of coaxial transmission line having an inner conductor of uniform longitudinal dimensions, a connection between said hollow metallic enlargement of said first resonant circuit and said inner conductor of uniform longitudinal dimensions of said second resonant circuit, and a set of third and fourthv resonant circuits similarly constructed to said vfirst and second resonant circuits,- and means for coupling together the hollow metallic enlargements of both sets of resonantcircuits. 15. A lter in accordance with'claim 14, characterized in this that said means is composed of additional resonant circuits in the form of sections of coaxial transmission line.

16. An electrical wave filter comprising a first resonant circuit composed of a section of coaxial transmission line whose inner conductor has two vhollow sections of different diameters, and a second resonant circuit composedof a section of coaxial transmission line located completely within the smaller diameter section of the inner conductor of said first circuit, whereby the inner surface of said smaller diameter section forms the outer conductor of said second resonant circuit, and means for capacitively coupling said resonant circuits together.

17. An electrical wave lter comprising a first resonant circuit composed of a section of coaxial transmission line having a hollow inner conductor, a second resonant circuit composed of a section of coaxial transmission line, and means coupling said two resonant circuits together comprising a third resonant circuit, vsaid third resonant circuit being located completely within the hollow inner conductor of said first resonant circuit.

18. An electrical wave filter comprising a iirst transmission line having a hollow inner conductor, a second resonant circuit composed cfa section of coaxial transmission line, and means coupling said two resonant circuits together comn prising a third resonant circuit, said third resonant circuit being located completely within the hollow inner conductor of said first resonant circuit and composed of coaxial metallic surfaces, the outer surface of which is the inner surface of the inner conductor of `said first `resonant BERTRAM mnvon.

resonant circuit composed of a section of coaxial l 

